The present invention relates to ultrasonic sensors for measuring the physical properties of materials within a space, and more particularly relates to a self test of such a sensor.
Known ultrasonic liquid level sensors installed in tanks include a sensor housing arranged around a gap or notch on the outside of the housing. When liquid level rises in the tank, the liquid flows into the gap and it is sensed by transmitting ultrasonic energy through the liquid in the gap. A pair of ultrasonic transducers or crystals in the housing transfer acoustic energy through the liquid in the gap via a resin transmission layer that bonds the crystals to the inside of the housing and provides acoustic impedance matching for increased transmission efficiency. Under harsh service conditions, the crystals can fail or the bonding of the transmission layer can fail and there is a need to test for such failures.
In addition to the transmission path through the fluid in the gap there is also a second transmission path between the crystals via the housing itself. In the past, sensing transmission along this second path around the gap through the metal housing has been a satisfactory means of testing the operation of the sensor when there is no liquid in the gap to provide transmission. Such a self test is described in U.S. Pat. No. 4,299,114 to Silvermetz et al.
With the use of non-metallic material for housings, such as polymers, resins, and the like (commonly called "plastics"), however, there are problems with using the housing itself as a test transmission path. These non-metallic materials change their sound transmission properties as a function of temperature. The variations can be large, and are quite unpredictable from one batch of plastic to the next, making it difficult to develop reliable electronic compensation for the variations.
The test signal transmitted through such plastic materials is unpredictable at temperature extremes giving a false indication that the sensor is defective. There is thus a need to provide a reliable test arrangement in ultrasonic sensors that does not rely on transmission of a test signal around the gap through the plastic housing.